1. Field of the Invention
This invention relates to improved drawn polyester films and processes for making the same. More particularly, this invention relates to monoaxially and biaxially drawn polyester films which are sufficiently transparent and have sufficient slip properties so that these films may be utilized and processed properly on high speed processing machines. This invention also specifically relates to improved drawn polyester films which can be used as support for magnetic recording tapes.
2. Description of the Prior Art
Polyester films and especially polyethylene terephthalate films are utilized for a great number of purposes since these films have excellent physical properties. Often, however, the physical properties which are desired for a particular application result in other properties of the polyester film which are sometimes contradictory to the property most desired. For example, the transparency and slipperiness of polyester films are all interrelated with surface roughness and, while it is often desirable to produce polyester films with good slip properties so that the polyester films have the desired processibility, these films also have an opacity caused by the same surface roughness which produces the excellent slip properties. Accordingly, these films are unacceptable for a number of uses wherein both high transparency and good slip properties are desired.
Most polyester film for various end uses is now prepared as a single layer polyester film with various coatings added thereto so as to modify the surface properties of the polyester film, such as in U.S. Pat. Nos. 3,595,736, 3,627,625, 3,645,822 and 3,632,666. The co-extrusion of a series of polyesters wherein at least one of the co-extruded layers has a polyester modifying additive contained therein has been suggested for U.S. Pat. No. 3,515,626. The process as disclosed in this patent, however, has not been completely successful in removing all of the problems regarding the contradictory properties of polyester film. In this regard, although incorporating a specific amount of crystallinity-promoting material; i.e., crystallizing sites, into the polyester before extrusion promotes the crystallinity of the polyester in one layer, while not promoting the crystallinity of the polyester in the other layers on drawing, the incorporation of a proper amount of these crystallization sites into the polyester is a very delicate operation requiring absolute precision so that the desired properties are produced. Furthermore, the utilization of these additives because they are added in addition to the catalysts and other materials during the polycondensation of the polyesters, occasionally hinders the polycondensation reaction itself and thereby degrades the physical characteristics of the polyester films produced thereby. This is especially true with regard to color and thermal stability of the polyester films.
Although it is desirable to produce polyester films which have excellent surface slip so that they can be wound at high speeds and processed properly without interference caused by static buildup, previously this surface slip could only be obtained with a concomitant reduction in transparency. Transparent polyester film is especially desirable when the end product, monoaxially or biaxially drawn polyester film, is to be utilized in the graphic arts or as a textile material. Furthermore, the reduction in surface smoothness is also necessary when the polyester film is to be utilized as a metallized film base for use in gold tooling or in the manufacture of magnetic tapes.
Although polyester films can be produced having acceptable slip and transparency properties when these films are oriented either monoaxially or biaxially, the slip properties are improved by the drawing, while the transparency is decreased. This is caused by an increase in the surface roughness of the film produced by the orientation.
Often, however, for many uses it is not necessary that the surface characteristics of the polyester film be identical on both sides of the film and the finished product may sometimes even be improved when the polyester film has asymmetrical surface characteristics; i.e., a film having different surface characteristics on each of its two surfaces.
One such product where different surface characteristics are desired is polyester film used as support for magnetic recording tapes.
Magnetic tapes are made, at present, of polyester film that is drawn monoaxially or preferably biaxially, sometimes even longitudinally superdrawn.
Magnetic tape supports have to provide excellent mechanical properties. Good tear strength, perfect elasticity that eliminates any risk of irreversible deformation in the zone of mechanical stresses, a modulus appropriate to winding and unwinding operations are also required of films for magnetic tape supports. They must also be dimensionally very stable, with respect to climatic variations as well as to mechanical stresses. Moreover, they must have good planar qualities, as well as uniform thickness. They must be devoid of local defects. All of these qualities are fairly consistently attained in the present state of the art. See, for example, U.S. Pat. Nos. 3,293,066, 3,397,072, 3,501,344, 3,554,798, 3,555,557 and 3,734,772 relating to magnetic recording tapes.
However, magnetic tape supports must additionally provide a satisfactory glaze; i.e., smoothness, on the surface intended to be coated with a magnetic composition; e.g., CrO.sub.2, Fe.sub.2 O.sub.3, etc. and the undersurface of the film, on the other hand, must have a coefficient of friction such as will readily ensure winding and unwinding. Additionally, the support must have good resistance to abrasion. Abrasion results from the tape rubbing on the rollers and other elements of the apparatus, and also results from rubbing of the two faces of the tape against each other during winding, etc.
According to known techniques, these surface properties are very difficult to harmonize and, therefore, respecting the requirements of satisfactory glaze and coefficient of friction, the prior art techniques could not provide completely satisfactory products.
These requirements are essential nevertheless, particularly for many new applications of magnetic recording tapes requiring very high winding/unwinding speeds and other stringent requirments.
When magnetic recording tapes are wound rapidly, it is required that the pack of tape be as flat as possible; i.e., individual layers of tape should not protrude appreciably from the rest of the pack. The main cause of unevenness is that, as the tape is wound, air is trapped between the layers of the tape, with the result that the tape floats on a cushion of air. The faster the tape is wound, the more air is trapped. In industry as well as in the studio, fast winding speeds are absolutely essential.
One prior art technique for providing a smooth surface/rough surface film for magnetic recording tapes was simply to roughen the undersurface of the film. This is usually effected by applying a coating in a thickness of about 5.mu., the coating being filled with pigments and/or dyes. By carefully adjusting the relative proportions of pigment and binder, it is possible to achieve a surface roughness of from 0.5 to 10.mu. in the dried coating. The statistically distributed irregularities in this layer penetrate the cushion of air and lead to early contact between the tape and the pack, with the result that floating of the tape on a cushion of air as it is wound is substantially avoided and the air can escape at the sides. However, the pronounced irregularities in the surface of this layer produce deformations in the magnetic tape during storage and these result in loss of contact between the magnetic layer and the recording or reproducing head when the tape is used.
U.S. Pat. No. 3,734,772, in an attempt to overcome some of these deficiencies, provides a pattern of raised dots having a high coefficient of friction on the undersurface of the film, but has not proven entirely satisfactory.
Though magnetic tapes intended for the recording of rather low frequency acoustic signals do not need a very fine glaze because the magnetic coatings that can accommodate themselves to these frequencies may be relatively thick and may be deposited without difficulty on rough supports whose undersurface presents an adequate coefficient of friction with easy winding, the situation is not the same where tapes for digital or analog recordings are concerned, such as those used at computer outputs. These must combine a fine glaze on the coated surface -- the coating being sufficiently thin for recording signals of a frequency that may reach 300 kiloHz -- with sufficient surface roughness for correct winding and unwinding.
Such tapes must be devoid of points of a certain height that could cause drop-out in recording or entail disturbance because of weakening of the signal. A compromise between such a surface state on the coated surface and the roughness of the undersurface that is sufficient to allow good film winding properties is extremely difficult to achieve. This is even more true because these tapes have to have good abrasion resistance which is essential to avoid formation of dust that could hamper reading of the tape. It is often necessary in practice to achieve an approximately satisfactory compromise result by coating the undersurface of the film support with a graphite or carbon base lubricant, but this is an obvious source of complication and difficulty, because of the need for good adherence of this coating and because of the risk of supplementary abrasion.
In the case of magnetic tape supports intended for analog signal recording for optical transmission; e.g., video recording, where signals with frequencies up to 5 to 7 megaHz have to be used for standard, and up to 20 megaHz for professional video recording, which requires extremely fine coatings, a compromise between the glaze of the coated face and the slip of the undersurface is not practical. In this case, it has previously been necessary that the undersurface receive a lubricant coating which cannot be devoid of a certain fragility.
It is within the above environment and background that the process and product of the present invention were developed.